Hematoporphyrin derivatives (HPD) is a complex mixture of porphyrins which has proved useful in cancer therapy and diagnosis. The primary goal of the proposed research is to identify, anc characterize, the tumor-localizing component(s) of HPD for the purpose of understanding the mode of the drug localization and the mechanism of the tumor-killing. Site-selection spectroscopy and resonance Raman spectroscopy, both based on tunable dye laser systems, will be used for the purpose. We also propose to compare photophysical properties (quantum yield of triplet formation, triplet state lifetime; and quantum yield and lifetime of fluorescence) of various porphyrins with their photosensitization efficiencies to deduce the factors that determine the efficiency of photodynamic action. These measurements are of fundamental importance to the improved therapeutic use of HPD and to the design of more efficient and less harmful drugs which could render photoradiation therapy a very powerful method of treating cancerous tissues.